freebsd-update: Correctly check if pkg(8) is present

On systems without pkg(8) installed, `command -v pkg` will return
success and falsely report that pkg(8) is present. Fix that by checking
via the `pkg -N` form.

This is missing from the final revision of D39695.

Reported by:	delphij
Reviewed by:	fernape, delphij
Fixes:		bc0c6c9 freebsd-update: Add check for kernel modules
Differential Revision:	https://reviews.freebsd.org/D45292

usr.sbin/freebsd-update/freebsd-update.sh

@@ -667,7 +667,7 @@ upgrade_check_kmod_ports() {
 	local report
 	local w
 
-	if ! command -v pkg >/dev/null; then
+	if ! pkg -N 2>/dev/null; then
 		echo "Skipping kernel modules check. pkg(8) not present."
 		return
 	fi